Asteroid

From Conservapedia

Asteroids are metallic, rocky bodies without
atmospheres that orbit the Sun but are too small to be
classified as planets. Known as "minor planets," tens of
thousands of asteroids congregate in the main
asteroid belt: a vast, doughnut-shaped ring located between
the orbits of Mars and Jupiter from approximately 2 to 4 AU
(186 million to 370 million miles/300 million to 600 million
kilometers). Gaspra and Ida are main belt asteroids.

Contents

Composition

Asteroids are thought by most modern astronomers to be primordial material
prevented by Jupiter's strong gravity from accreting into a
planet-sized body when the solar system was born 4.6 billion
years ago. It is estimated that the total mass of all
asteroids would comprise a body approximately 930 miles
(1,500 kilometers) in diameter -- less than half the size of
the Moon.

Sizes

Known asteroids range in size from the largest --
Ceres, the first discovered asteroid in 1801 -- at about 600
miles (1,000 kilometers) in diameter down to the size of
pebbles. Sixteen asteroids have diameters of 150 miles (240
kilometers) or greater. The majority of main belt asteroids
follow slightly elliptical, stable orbits, revolving in the
same direction as the Earth and taking from three to six
years to complete a full circuit of the Sun.

Understanding

Our understanding of asteroids has been derived from
three main sources: Earth-based remote sensing, data from
the Galileo flybys, and laboratory analysis of meteorites.
Asteroids are classified into different types according to
their albedo, composition derived from spectral features in
their reflected sunlight, and inferred similarities to known
meteorite types. Albedo refers to an object's measure of
reflectivity, or intrinsic brightness. A white, perfectly
reflecting surface has an albedo of 1.0; a black, perfectly
absorbing surface has an albedo of 0.0.

Categories

The majority of asteroids fall into the following three
categories:

C-type (carbonaceous): Includes more than 75 percent

of known asteroids. Very dark with an albedo of 0.03-0.09.
Composition is thought to be similar to the Sun, depleted in
hydrogen, helium, and other volatiles. C-type asteroids
inhabit the main belt's outer regions.

S-type (silicaceous): Accounts for about 17 percent

of known asteroids. Relatively bright with an albedo of
0.10-0.22. Composition is metallic iron mixed with iron- and
magnesium-silicates. S-type asteroids dominate the inner
asteroid belt.

M-type (metallic): Includes many of the rest of the

known asteroids. Relatively bright with an albedo of 0.10-
0.18. Composition is apparently dominated by metallic iron.
M-type asteroids inhabit the main belt's middle region.

Meteorites

The most common meteorites, known as
ordinary chondrites, are composed of small grains of rock
and appear to be relatively unchanged since the solar system
formed. Stony-iron meteorites, on the other hand, appear to
be remnants of larger bodies that were once melted so that
the heavier metals and lighter rocks separated into
different layers.

Controversy

A long-standing scientific debate exists over whether
the most common asteroids -- the S-types -- are the source
of ordinary chondrites. Spectral evidence so far suggests
that the S-type asteroids may be geochemically processed
bodies akin to the stony-irons. If S-types are unrelated to
ordinary chondrites, then another parent source must be
found. If the two are related, then scientists need an
explanation for why they aren't spectrally similar.

Near Earth Asteroids

Asteroids with orbits that bring them within 1.3 AU
(121 million miles/195 million kilometers) of the Sun are
known as Earth-approaching or near-Earth asteroids (NEAs).
It is believed that most NEAs are fragments jarred from the
main belt by a combination of asteroid collisions and the
gravitational influence of Jupiter. Some NEAs may be the
nuclei of dead, short-period comets. The NEA population
appears to be representative of most or all asteroid types
found in the main belt.

NEAs are grouped into three categories, named for
famous members of each: 1221 Amor, 1862 Apollo, and 2062
Aten:

Amors: Asteroids which cross Mars' orbit but do not

quite reach the orbit of Earth. Eros -- target of the NEAR
mission -- is a typical Amor.

Apollos: Asteroids which cross Earth's orbit with a

period greater than 1 year. Geographos represents the
Apollos.

Atens: Asteroids which cross Earth's orbit with a

period less than 1 year. Ra-Shalom is a typical Aten.

NEAs are a dynamically young population whose orbits
evolve on 100-million-year time scales because of collisions
and gravitational interactions with the Sun and the
terrestrial planets. Approximately 250 NEAs have been found
to date, probably only a few percent of their total
population. The largest presently known is 1036 Ganymed,
with an approximate diameter of 25.5 miles (41 kilometers).
Estimates suggest at least a thousand NEAs may be large
enough -- 0.6 mile (1 kilometer) or more in diameter -- to
threaten Earth.

Strikes and near misses

The Hoba meteorite

Many bodies have struck Earth and the Moon in the past,
and one widely accepted theory blames the impact 65 million
years ago of an asteroid or comet at least 6 miles (10
kilometers) in diameter for mass extinctions among many
lifeforms, including the dinosaurs. Other theories suggest
that the chemical building blocks of life and much of
Earth's water arrived on asteroids or comets that bombarded
the planet in its youth.

On June 30, 1908, a meteoroid (estimated 100
meters, or around 330 feet) in diameter entered the Earth's atmosphere, exploding over the remote region of
Tunguska in Siberia, devastating more than half a million
acres of forest.

One of the most recent close calls occurred
on March 23, 1989, when an asteroid 0.25-mile (0.4-
kilometer) wide came within 400,000 miles (640,000
kilometers) of Earth. Surprised scientists estimated that
Earth and the asteroid -- weighing 50 million tons and
traveling at 46,000 miles/hour (74,000 kilometers/hour) --
had passed the same point in space just six hours apart.

On February 15, 2013 an meteor exploded in the Ural Mountains.
The shock wave shattered many windows, causing over 1,000 people to be injured.[1][2]